Alternating-current amplifier



A; V. LOUGHREN Jan. 15, 1929. 1,699,110

ALTERNATHJG CURRENT AMPLIFIER Filed Oct. 4. 41927 2 Sheets-Sheet 2 Current te Volts Fig.5.

Inventor:

ATthLiT V Loughre'n,

His Attorney.

Patented Jan. 15, 1929.

UNITED .sT rEs PATENT OFFICE.

ARTHUR V. 'LOUGHBEN, OI SQHENECTADY, NEW YORK, ASSIGNOB 'I'O TRIO OOHPANYfA CORPORATION OF NEW YORK.

onnmr. unc- ALTlRNATING-CUBRENT AIPLIIIER.

Application filed October 4, 1927. Serial No. 223,861.

1 current power than has heretofore been practicable. Y I I Heretofore in the operation of circuits of this type it has been customary to ad ust the anode current and voltage of the electron 1 discharge devices at a value such that their product was equal to the rated anode d1ss1- pation of the tube. This is particularly true where the device is employed for the purpose of producing large amounts of alternating current power in its output circuit and in other applications the normal no load anode current value lies at the center of the straight portion of the characteristic curve of the 'tube. Since the maximum value of the alternating current component of the anode current can never exceed the no load anode current it can readily be shown that the maximum value of alternating current output can never exceed one half of the rated anode dissipation and in practical cases is generally below three tenths of this value. This practical value results from the fact that as the anodecurrent approaches zero it no longer reproduces'the impressed wave and distortion results. To eliminate this it is customary to operate the tubes with a negative grid bias such that during the negative half of the impressed alternating current wave the plate current does not fall below the lower limit of the straight portion of the anode current-voltage curve. .This rated no load anode dissipation and the inci dent grid bias so limited the device that notwithstanding that an alternating current input having perfect regulation was available still the alternatin could never exceed t e value mentioned.

I have found that by reducing the grid voltage ,to a value such that substantially no current flows in the anode circuit during no load conditions I am able not only to reduce the no load losses substantially to zero, but also to increase the-distortionless alternating current power output to, in many cases, several times the rated anode dissipation.

current output power v The novel features which I believe to be characteristic of my invention will be set forth in the appended claims. My invention itself, however, both as to its organization and method of operation may best be understood b reference to the following description ta en in connection with the accompanying drawings, in which Fig. 1 represents a circuit with which my inven; tion may be emplo ed and which is shown as a modulator. Figs. 2, 3, 4 and 5 represent curves indicative of the operation of my invention.

With reference to Fig. 1, 1 and 2 represent a pair of electron dischar e devices the grids of which are connecte to opposite extremities of the secondary winding of an input transformer 3, an intermediate point of which is connected through the 'd biasing battery 4 to the cathodes of bot devices. The anodes of these devices are likewise connected to opposite points on the primary winding of the output transformer 5 an intermediate point of which is connected to the positive side of the potential Source 6. The negative side of this source is connected to the cathodes of both devices. For the purpose of impressing upon the grids of these devices an alternating electromotive. force which is to be amplified I have shown the primary winding of the transformer 3 connected to a source ofpotential 7 and a microphone 8 through the intermediary of suitable additional am lifying means 9. 10 represents an electron ischarge device which is employed asan oscillation generator and which .is provided with the usual oscillatory circuit 11 which is ada ted to supply high frequency currents to the load circuit 12 which is represented in the form of an antenna. Anode potential is supplied to the device 10 through a circuit including ground source of otential 6, secondary winding of the trans ormer 5, radio' frequency choke coil 13, space between the anode and cathode of the device 10 and ground. As thus arranged alternating currents produced by the source 8 are amplified by the devices 1 and 2 and caused to modulate the currents produced by the oscillation generator 10.

In practicing my invention I adjust the potential source 4: in such a way that the grids are maintained sulficiently negative with respect to the cathodes tosubstantially preclude the flow of anode current from these devices, or at. least, to preclude the generation of third harmonics in a manner later to be set forth. The operation of the device, with the grid bias thus adjusted, may best be understood by reference to Fig. 2 where I have plotted at 14 a portion of the grid voltage anode current characteristic of the de-. vice. It will be seen that with the grid biased negatively to a value equal to a the anode current will be very small, this value being represented by b. If.v an alternating current wave having the form shown at 15 and an amplitude'equal to a be impressed upon the grid of one of these devices theanode current will take the form shown at .16 reaching a very high value during the positive half cycle and zero during the substantial portion of the negative half cycle.

Since the anode current curve of the opposite electron discharge device is 180 displaced in phase with respect to that of the first der 155. section with the zero axis.

vice this may be represented by the curve 17. It will be seen that while each of these curves 16 and 17 represents a badly distorted wave that the sum of the two as combined in the secondary winding of the output transformer represents an undistorted sine wave. Thus the positive half cycles of the input wave are amplified by one of the electron dischargedevices and the negative half cycles are amplified by the other electron discharge device, these waves being combined to produce an undistorted sine wave in the secondary winding of the output transformer. For this reason I prefer to term my system a push-push amplifier as distinct from the well-known push-pull amplifier of the prior art. l.

While in Fig. 2 I have shown the characteristics of the device with a no load plate current equal to a small value flowing in the output circuits of each device it will, of course, be understood that in many cases an undistorted output may be had even though no plate current flows under no load conditions. In Fig. 3 I have shown the form of anodecurrent curve which frequently results when the grid isso adjusted that no anode current flows under no load conditions. It will be seen that the curves 16 and 17., which correspond to the similarly numbered curves of Fig. 2 do not add up to produce a sine wave form near the point of inter- I have found thatthis form of distortion of the output wave is represented by a pronounced third harmonic and other odd harmonics in the secondary winding of the output transformer and that this may be completely eliminated by permitting a small amount'of no load current to flow. However, this no load anodecurrent value, due to the additive effects of the two distorted waves, represents a value considerably below the straight poracteristic curve, and a no load anode loss which is negligible.

Further advantages of my device are apparent from a consideration of Figs. 4 and 5. In each of these figures I have plotted the anode voltage anode current characteristics of a standard fifty watt tube of commercial construction with different bias voltages upon the grid. In Fig. ithe tube employed was one having a ratio AE I ==K where AE =an increment of plate voltage change corresponding to an increment, AE of grid voltage change, the plate current I remaining constant. In Fig. 5 the tube employed was one having this ratio equal to about 25.

Referring to Fig. 4 the areaof triangle ABC is indicative of the power output of a single tube connected in the usual push pull circuit'and operating with a no load anode current equal to CD. -Due to the rated anode dissipation of the tube this value cannot be increased. During maximum alternating current in the output circuit the grid must swing between values represented by the curves which include points A and E corresponding respectively to zero grid volts and a minimum value determined by the curvature of the anode current .anode voltage characteristic which will not give more than permissible distortion. The amplitude of the alternating current in the plate circuit will then be equal to BC, that of the anode voltage will be equal to AB andthe load resistance will be represented by the slope of the line AC. The triangle FGD represents the output power from a pair of the same tubes operated with the grids biased to cut off in accordance with my invention anode circuit resistance is equal to the internal anode resistance. Graphically this means that the lines FD and AG should be of the same slope as the anode voltagecurrent characteristic but opposite in sign. It will be seen that this is approximately attained in the case of the line FD, indicative'of the operation of my invention, Whereas a. similar result is impracticable when the usual grid bias is employed.

tionof the anode current grid voltage char- I have found that a similar advantage All . may be had.

may be attained b my invention when tubes having a hig are employed but that it is not so pronounced except as the grid. of the tube is driven positive during a portion of one half cycle of the wave. This result is'illustrated by the triangles FGD and ABC of Fig. 5 which correspond to the similarly lettered triangles of Fig. 4, the triangle of Fig. 5 being indicative of the operation of a tube having a ratio =25. Of course, when the grids of the tubes are driven positive it is essential to a distortionless output that the voltage of the input to the tube should not vary appreciably with the flow of grid current. In the case shown in Fig. 5 the tube, the operation of which the triangles FGD and ABC are indicative, operated with very approximately the same internal heating whereas the alternating current output represented by the triangle FGD is far greater than that represented by triangle ABC. Likewise it will be noted that in the former case the tube may be operated ,into a roper external impedance for maximum e ciency since the slope of the line FD is practically equal to that of the anode current voltage characteristics.

Since the advantages resulting from, the practice of my invention vary greatly with different types of tubes and since Figs. 2 to 5 are only indicative of the operation of particular types it will of course be understood, that these'figures are presented only as a general indication of the results which tubes of low rated capacity and of low ,1. I have found that the resulting advantage is attained largely by virtue of the fact that the grids may be driven positive. I have found the same true with high 1:. tubes of large capacity. This, however, is a distinct advanta e since when using the usual grid bias it is generally impracticable to drive the grids positive to an considerable degree due to distortion w ich results at the lower portion of the anode current grid voltage curve, such as at the points E in Figs. 4 and 5. I also find that greater advantages resulting from my invention may be expected when practiced in connection with tubes of large capacity.

While I have shown and described a single embodiment of my invention it will, of course, be understood that I do not wish to be limited thereto, since modifications may be .madein the circuit arran ement and in the instrumentalities employe without departing from the spirit and scope of my invention as set forth in the appended claims.

What I claim as new and desire to secure by Letters Patent of the United States, is

1. In combination, a pair of three element electron discharge devices, means connecting For example in the case ofsubstantially the entire period of alternate alternating half cycles of the alternating current wave when alternating potential is impressed upon the grids. r

2. In combination, a pair of electron dis charge devices, means connecting said discharge devices in push pull relation including an input circuit and an output circuit, means for impressing an alternating potential upon said input circuit for reproduction by said devices in said output circuit, a source of direct current potential connected to bias negatively the grids of said devices during said reproduction the value of said biasing potential being determined by the substantial absence of direct current in the anode circuits of said devices when no alternating potential is impressed upon the grids, said connecting means including a non-resonant output circuit and means for supplying energy from each device to said circuit thereby to combine the output currents from said devices in said non-resonant circuit to reproduce the impressed alternating current wave.

3. In combination, a pair of electron dissaid dis charge devices, means connecting charge devices in push pull relation including means for impressing an alternating potential upon the control elements thereof, means for impressing a negative direct current potential on each of said control elements of value such that the flow of current from each of said devices during a substantial portion of alternate halfcycles of.

the impressed alternating current wave is precludedand such that each device reproduces the other halfcycle of the impressed ioo wave with substantially true wave form, and

means included in saidconnecting means for combiningthe current flowin from said devices during the last mentione portion of the alternating current cycle to reproduce the impressed alternating current wave.

4. In combination, a pair. of electron discharge devices, means connecting said devices inpush pull relation including an input circuit, means for impressing an alternating current potential upon said input circuit for amplification by said devices and a source of potential connected to biasthe grids of said devices during said amplification sufficiently to reduce t anode current to a value lying in the lower curved portion of the anode current grid voltage characteristic, said connecting means being adapted to combine the output from said devices to produce an alternating current wave.-

5. In combination, a pair of electron dis charge devices, means connecting said devices in push pult relation including an input circuit and an output circuit, means for impressing an alternating current potential upon said input circuit for reproduct-ion by said devices in said output circuit and a source of potential connected to bias the grids of said devices during said reproduction sufficiently to reduce the anode current to a value lying in the lower curved portion of the anode current grid voltage characteristic, said value being determined by the absence of odd harmonics of the impressed wave in the output circuit, said connecting means being adapted. to combine the output from said devices toreproduce the impressed. alternating current Wave.

6. In combination, a pair of space discharge devices having control electrodes, means connecting said devices in push pull relation including means for impressing upon the control electrodes thereof alternating current potential for amplification thereby, means for impressing a steady biasing potential upon said grids during said ampli cation of a value such as to cause a small change in anode current from each of said devices during one half cycle of the alternating current wave and com aratively greater change in anode current uring the opposite half cycle, said connecting means being adapted to combine the output from both of said devices to produce an alter nating current wave.

7. In combination, a pair of space discharge devices having control electrodes, means connecting said devices in push pull relation including means for impressing upon the control electrodes thereof alternating current potential for reproduction thereby, means for impressing a steady biasing potential upon said grids during said reproduction of a value such as to cause one at said devices to repeat one half cycle 1 of the alternating current Wave and the hand this 3d day of October, 1927.

. ARTHUR V. LOUGHREN.

\ In witness whereof, I have hereto set my 

